Heat-insulating layer made of complex perovskite

ABSTRACT

A heat-insulating material has a melting point above 2500° C., a thermal expansion coefficient in excess of 8×10 −6  K −1 , and a sintering temperature greater than 1400° C. It has a perovskite structure of the general formula A 1+r (B′ 1/3+x B″ 2/3+y )O 3+z  where
         A=at least one element of the group (Ba, Sr, Ca, Be),   B′=at least one element of the group (Mg, Ca, Sr, Ba, Be),   B″=at least one element of the group (Ta, Nb),   r, x, and z≠0, and   −0.1&lt;r, x, y, z&lt;0.1;
 
or of the general formula A 1+r (B′ 1/2+x B″ 1/2+y )O 3+z  where A and B″ are as above and
   B′=at least one element of the group (Al, La, Nd, Gd, Er, Lu, Dy, Tb), and   −1.0&lt;r, x, y, z&lt;0.1.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US national phase of PCT applicationPCT/DE03/01924 filed 10 Jun. 2003 with a claim to the priority of Germanpatent application 10226295.0 itself filed 13 Jun. 2002 whose entiredisclosures are herewith incorporated by reference.

FIELD OF THE INVENTION

The invention relates to a heat-insulating layer which is made from acomplex perovskite.

STATE OF THE ART

To increase the efficiency of stationary and flying gas turbinesincreasingly higher gas temperatures are today required in suchmachines. For this purpose components of the turbine are provided withheat-insulating layers (HIL) which as a rule are comprised of yttriumstabilized zirconium oxide (YSZ). An adhesion promoting layer (APL) of aMCrAlY alloy (M=Co, Ni) or an aluminide layer between the substrate andthe heat-insulating layer serves mainly for protecting the substrateagainst oxidation. With this system, surface temperatures of the turbineelements up to 1200° C. can today be realized.

A further increase to above 1300° C. is desirable but however has notbeen realizable with the workpieces used to date, especially with YSZ.The zirconium oxide deposited by plasma spraying or electron beamvaporization undergoes at temperatures above 1200° C. a phasetransformation as well as accelerated sintering processes which can giverise to damage to the layer within the operating time. At the samethermal conductivity of the heat-insulating layer and the same layerthicknesses, higher surface temperatures also bring about highertemperatures in the adhesion promoting layer and the substrate. Thesetemperature increases also contribute to an accelerated deterioration ofthe bond between the materials.

For these reasons there is a world wide search for new materials whichcan replace the partly stabilized zirconium oxide as a material for aheat-insulating layer.

From DE 100 56 617 (U.S. Pat. No. 6,821,656) it is known to userare-earth perovskites as heat-insulating layers where La, Ce, Pr or Ndare present in the A position and Er, Th, Yb or Lu are present in the Bposition. Such perovskites are characterized by a high melting pointwhich lies, depending upon the material above about 1800° C. andespecially even above 2000° C. Up to the region in which the materialreaches its melting temperature, such a material shows no phasetransformation and thus can be used for corresponding purposes,especially as a heat-insulating layer. A further characteristic of thisaforementioned perovskite is its thermal expansion coefficient oftypically greater than 8.5×10⁻⁶ K⁻¹. Furthermore, its reduced thermalconductivity of less than 2.2 W/mK is advantageous for its use as aheat-insulating layer.

Perovskites with these characteristics function especially well asheat-insulating layers on a metal substrate since the thermalcoefficient of expansion is matched and mechanical stresses between thetwo materials upon a temperature increase is limited and the reducedthermal conductivity usually limits overheating of the substrate.

Furthermore, a complex perovskite family of the general formula A²⁺(B²_(1/3) ⁺B⁵⁺ _(2/3))O₃ is known. These perovskites, because of theirtemperature equalizing effect and their capacity to serve as low lossdielectrics have found use in many wireless communication devices (L.Dupont, L. Chai, P. K. Davies: “A- and B-site order in(Na_(1/2)La_(1/2))(Mg_(1/3)Ta_(2/3))O₃ perowskites”; A. S. Bhalla, R.Guo, R. Roy, “The perowskite structure—a review of its role in ceramicscience and technology”, Mat. Res. Innovat. (2000) Vol. 4., 3-26).

OBJECT OF THE INVENTION

The object of the invention is to provide a heat-insulating material fora heat-insulating layer which fulfills the requirements of a low thermalconductivity, a high thermal coefficient of expansion and a highsintering temperature simultaneously with a good phase stability up totemperatures in excess of 1300° C. Furthermore it is an object of theinvention to provide thermally stressed components with such a thermalinsulating layer.

SUMMARY OF THE INVENTION

The object is achieved with a heat-insulating material for aheat-insulating layer comprising a thermal protective layer of aheat-insulating layer with a complex perovskite pressure with a meltingpoint above 2500° C. with a thermal expansion coefficient of at least8×10⁻⁶ K⁻¹ and with a sintering temperature of greater than 1400° C.This heat-insulating material is characterized by a complex perovskitestructure in accordance with the following general formulaA_(1+r)(B′_(1/3+x)B″_(2/3+y))O_(3+z).

In this formula A represents at least one element from the group Ba, Sr,Ca, Be, B′ represents at least one element of the group Mg, Ba, Sr, Ca,Be and B″, represents at least one element of the group (TA, Nb).Alternatively the heat-insulating material can also have a compositionaccording to the following formula:A_(1+r)(B′_(1/2+x)B″_(1/2+y))O_(3+z).

In this case, A can represent an element of the group Ba, Sr, Ca, Be, B′can represent at least one element of the group Al, La, Nd, Gd, Er, Lu,Dy, Tb. For B″ at least one element for the group (Ta, Nb) is selected.For both of the aforementioned compositions, such compounds should beincluded within the framework of this invention which have a slightdeviation from the stoichiometry, than is such than −0.1<r, x, y, z<0.1.

It has been found that by contrast with many other materials of theperovskite class, these heat-insulating materials have an ordered formwith a layered structure in which the layers of B′ and B″ alternatecorresponding to the stoichiometry. Also three or more atoms in the Bplaces, again strictly maintaining the stoichiometry, are possible, asis a mixture of the atoms in the A places. A certain deviation from thestoichiometry in the range of up to 5% is tolerable.

In addition, additives in an amount of several percent of such foreigncations which do not have ionic radii deviating excessively from thoseof the original cations are also possible.

The heat-insulating material has advantageously a high coefficient ofthermal expansion in excess of 8×10⁻⁶/K and a reduced tendency tosinter. Typical sintering temperatures of these materials usually lieabove 1400° C.

All heat-insulating materials have a high phase stability to above 1350°C. The thermal conductivity of these perovskites is also highlysatisfactory for their use as heat-insulating materials since thethermal conductivity of less than 3 W/m/K is especially low andsatisfactory.

In addition, the melting points of these heat-insulating materialsaccording to the invention usually lie above 2000° c. and in part alsoabove 2500° C. In addition, with these classes of materials there arisesan average to large difference between the cation masses whichadditionally contributes to a reduction in the thermal conductivity.

All of these characteristics make the aforedescribed materials highlysuitable for use as heat-insulating materials.

An especially advantageous representative of the group of theaforementioned heat-insulating materials is Ba(Mg_(1/3)Ta_(2/3))O₃.Other compounds which have been found to be especially suitable areSr(Al_(1/2))Ta_(1/2))O₃, Ca (Al_(1/2))Nb_(1/2))O₃,Sr(Sr_(1/3)Ta_(2/3))O₃ or Sr(La_(1/2)Ta_(1/2))O₃.

A heat-insulating layer made from these materials has as rule a meltingpoint of about 3000° C. and an extremely low sintering tendency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph depicting temperature, time, and the coefficient ofthermal expansion for an embodiment of the invention.

SPECIFIC DESCRIPTION

In the following, the subject of the invention is described in greaterdetail in connection with an exemplary embodiment and without limitingthe subject of the invention thereto.

1. Characteristics of the Heat-Insulating Material.

An especially suitable heat-insulating layer is obtained advantageouslyfrom the heat-insulating material with the composition:Ba(Mg_(1/3)Ta_(2/3))O₃ which is fabricated by a solid state reaction ofBaCO₃, MgO and Ta₂O₃. After pressing the material is sintered at 1600°C. for several hours without noticeable shrinkage occurring. Thismaterial is thus suitable fur use as a heat-insulating layer where a lowtendency of the material to sinter is desirable.

Advantageously, it is possible to make Sr(Al_(1/2)Ta_(1/2))O₃ or Ca(Al_(1/2)Ta_(1/2))O₃ from Al₂O₃ and Ta₂O₃ and SrCO₃ or CaCO₃. In generalthe elements barium, strontium, and calcium, preferably as carbonatesand the remaining elements preferably as oxides can be provided in amixture. The amounts of the individual compounds are selected so thatthey correspond to the aforementioned stoichiometric composition. Usinga solid state reaction, the desired perovskite is obtained. Compositionswith a slight deviation from the stoichiometry as previously mentionedcan be made also by a suitable choice of the starting amounts.

With the thus produced Ba(Mg_(1/3)Ta_(2/3))O₃ a dilatometer test iscarried out. The FIGURE shows the results for this material. Theheat-insulating material has a coefficient of thermal expansion at 1000°C. of 10.4×10⁻⁶/K. This value is comparable with those for the standardmaterial YSZ and is highly advantageous for a use of the material as aheat-insulating material.

2. Production of a Heat-Insulating Layer System (HIS).

The heat-insulating material produced by the solid state reactionunder 1. with the composition Ba(Mg_(1/3)Ta_(2/3))O₃ can be granulatedby spray drying and then processed by a subsequent thermal spray processlike atmospheric plasma spraying (APS) to an HIS system. In this case,nickel-based or cobalt based alloy are provided by vacuum plasmainitially with an MCrAlY layer (M=Co, Ni) adhesion promoting layer(thickness of the layer about 50 to 500 μm). Then by atmospheric plasmaspraying (APS) the heat-insulating layer is applied from the materialaccording to the invention in a layer thickness of about 50 to 3000 μm.Alternatively, one can also make a two layer thermal insulating layer inthat a first layer is applied from the YSZ material and the upper layeris then applied from the heat-insulating material (for exampleBa(Mg_(1/3)Ta_(2/3))O₃ (BMT)) by deposition.

1. In combination: a thermally stressed turbine component, and aheat-insulating layer overlying a surface of the turbine component andhaving a perovskite structure of the general formulaA_(1+r)(B′_(1/3+x)B″_(2/3+y))O_(3+z) in which: A=at least one element ofthe group (Ba, Sr, Ca, Be), B′=at least one element of the group (Mg,Ca, Sr, Ba), B″=at least one element of the group (Ta, Nb), and −0.1<r,x, y, z<0.1.
 2. The combination defined in claim 1 wherein theheat-insulating layer has a composition of the formula Ba(Mg_(1/3)Ta_(2/3))O₃.
 3. The combination defined in claim 1, furthercomprising between the surface of the component and the heat-insulatinglayer: an intermediate layer of ceramic glass or metallic material. 4.The combination defined in claim 3, wherein the intermediate layercomprises a MCrAlY alloy where M=Co or Ni.
 5. A method of protecting athermally stressed turbine component, the method comprising: applying toa surface of the turbine component a heat-insulating layer having aperovskite structure of the general formulaA_(1+r)(B′_(1/3+x)B″_(2/3+y))O_(3+z) in which: A=at least one element ofthe group (Ba, Sr, Ca, Be), B′=at least one element of the group (Mg,Ca, Sr, Ba), B″=at least one element of the group (Ta, Nb), and −0.1<r,x, y, z<0.1.